First, spending legislation: Passing a multi-month omnibus bill, or a shorter term package, must happen this weekend before the government runs out of money. It's also likely key to Democrats' hopes of passing their other major outstanding priorities including START and repealing Don't Ask, Don't Tell.
The brinksmanship over spending is complicated, but sufficed to say Harry Reid won't have a huge margin if he hopes to pass the omnibus.
Once that problem is solved, Dems will have to decide if and how to pass the rest of their agenda. Ratifying START (or any treaty) requires a two-thirds majority of all members present and voting. That's 67 votes if all 100 members are there, and 66 if one is absent. They got 66 votes on the procedural motion to debate the treaty.
On Don't Ask, Don't Tell, 61 senators claim to support a standalone measure to deep six that policy. But it will take 60 to break a GOP filibuster. And a small handful of those 61 have caveats: they want tax and spending issues completely resolved before agreeing to vote for repeal.
Reid has threatened to keep the Senate in session all the way until January 5, when the 112th Congress starts if that's what it takes to get all this done. His margin for error just got smaller.
[Ed note: This post has been update to clarify the process of treaty ratification.]